This invention relates to a method and an apparatus for determining the wheel/rail adhesion for a railway vehicle on a railway track, and for enhancing that adhesion.
If the wheel/rail adhesion for a railway vehicle is insufficient, then the wheels may slip when the vehicle attempts to accelerate or to brake. This has been a recognised problem for many years, and the application of sand onto the rails has long been used to enhance adhesion. The sand may be applied manually, or it may be supplied using compressed air, for example in response to a measured value of the deceleration of the wheels (as in GB 1 349 145 (Girling)). It has also been suggested, in EP 0 217 636 (Mitsubishi), that the quantity of sand particles dispensed may be adjusted in accordance with the speed of the vehicle. An alternative way of dealing with this problem during braking is to monitor both vehicle speed and wheel speeds, and if slipping is detected to reduce the brake pressure, for example to permit a controlled level of slip; for example the wheels might be allowed to slip so that they turn at a rate say 10 or 15 percent less than that corresponding to the vehicle""s speed. Such wheel slide protection systems are currently in use. Controlled slipping of the wheels optimises the braking force, and also tends to remove contamination from the rail head.
According to the present invention there is provided a method for determining the wheel/rail adhesion for a railway vehicle, the method comprising monitoring operation of the braking system when the brakes are applied, and hence determining the value of the adhesion.
In one embodiment the vehicle incorporates an accelerometer, and the method comprises comparing the deceleration measured by the accelerometer to the deceleration demanded by the driver, and determining from any discrepancy between those values of deceleration the value of the adhesion. In an alternative embodiment the vehicle incorporates a wheel slide protection system which modulates the pressure in a brake cylinder in response to detection of wheel slip, and the method comprises determining the pressure supplied to the brake cylinder, and determining from that pressure the value of the adhesion.
The value of adhesion may be indicated to the driver of the vehicle. If exceptionally low adhesion is identified in this way, a warning message may be transmitted to other vehicles on the line, and to signalling staff. Where the method involves determining the pressure supplied to the brake cylinder, the pressure may be measured, or alternatively may be calculated, for example by a computer in the wheel slide protection system.
The present invention also provides a method for enhancing the wheel/rail adhesion for a railway vehicle by applying adhesion-enhancing particles to the wheel/rail interface at a rate which is adjusted in accordance with the value of the adhesion so determined.
The adhesion-enhancing particles may be particles of sand or quartz or other suitable material and may be supplied dry, using compressed air. The rate at which the particles are supplied may also be adjusted in accordance with other measured values, in particular the vehicle""s speed, and preferably also the vehicle""s deceleration. Furthermore the rate at which particles are supplied may also be adjusted in accordance with the desired deceleration, as specified by the driver. Currently in the United Kingdom trains which dispense sand do so at a standard rate of 2 kg/min. Adjusting the rate of supply of particles in accordance with vehicle speed to higher values than that, at higher speeds (say above 50 km/hr), will enable improved braking performance to be achieved, while reducing the rate of supply to lower values than that, at low speeds (say below 15 km/hr), will minimise the problems caused to the track circuit.
The invention also provides an apparatus for enhancing the wheel/rail adhesion for a railway vehicle, by adjusting the rate of supply of adhesion-enhancing particles in the manner defined above. The rate of supply of particles may be continuously variable, or alternatively the rate of supply may be adjustable in steps. It will be appreciated that any such apparatus is desirably arranged so that if the adjustment fails, the apparatus should revert to dispensing the particles at a constant rate (e.g. 2 kg/min).
The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawing which shows a flow diagram for a train braking system.